This invention relates to a method and a device for dispersing material in a dry state or suspended in water to an aerosol or other three-phase system.
The invention is generally applicable and can be applied to all kinds of material, but it is especially suitable to be applied to fibrous materials, which may be difficult to disperse in gas flows.
A well-dispersed fibre aerosol is a prerequisite for rendering it possible for fibres of different kinds to be mixed in a gas-dynamic way.
When, for example, the fibres are to be dried in a flash drier, the greatest heat transfer surface is obtained when the fibres are entirely exposed. A large surface in its turn permits a lower difference in temperature between drying gas and drying material, thereby improving the efficiency degree of the drier.
All of the shredder types commercially available today and employed for dispersing fibres in flash drying plants are of a mechanical type, i.e. the papermaking pulp is disintegrated by shearing between mechanical devices. NIRO ATOMIZER sells a roll with spikes, SUNDS passes the pulp through a rotating pin wheel, and DEFIBRATOR offers disc refiners.
All these types of shredders have in common that, at the same time as they produce the disintegrating tensile and expansion forces, they also give rise to sintering compression forces. The pulp shredded for the flash drier includes single fibres, undefibrated flakes and compressed fibre packages.
The free fibres dry within some seconds in the flash drier, but the larger fibre flakes require a drying time in the drier of almost one minute. This implies that the free fibres are over-dried, their dry solid content is 100 percent while the average material has a dry solid content of 90 percent. Over-drying implies, in addition to a lower efficiency degree, also a deterioration in quality. The free fibres form spirals, and their surface gets hard.
The compressed fibre packages, which are made permanent in the drier, form knots, which are almost impossible to pulp. This problem is particularly troublesome with birch pulp and some other hardwood pulps which, therefore, today are not flash dried at all.
When the number of free fibres can be increased at the shredding operation, the drying temperature can be lowered. This reduces the effect of making the fibre packages permanent and, besides, decreases the number of fibre packages to become permanent.
The present invention relates to a gas-dynamic method of shredding papermaking pulp, hereinafter called jet shredding. The utilization of a gas as shredding medium implies, that the strongest compressing forces disappear, because gases are compressible and, therefore, have a certain "air cushion effect". In order to achieve highest possible efficiency, the following requirements must be met:
1. Great difference in velocity between gas and material. The material then is exposed to strong acceleration forces, which upon acceleration of the material tear off fibres. PA0 2. Lower static pressure on the gas than in the fibre material. The fibre material then tends to expand apart and thereby facilitates defibration. PA0 3. High temperature of the gas. The material is easier to disperse at increasing gas temperature, because the fibres are held together by the capillary forces of the water, which decrease at increasing temperature and are completely gone at the critical water temperature.